1. Field of the Invention
The present invention is related to DC-DC conversion technology, specifically, the power system where rechargeable batteries are used as auxiliary power source, in which the voltage of the rechargeable batteries are boosted to high voltage system (or bus bar) to provide power for emergency or capacity adjustment, or in reverse charges the rechargeable batteries.
2. Description of the Prior Art
Clean energy source becomes an important subject due to oil shortage crisis. New hybrid car can increase mileage and reduce oil consumption. Due to frequent switching between battery power and engine power in order to maximize fuel efficiency, high efficiency bi-directional converter becomes a critical element. Traditionally, serially connected rechargeable batteries are used in order to reduce voltage gap between battery and appliance and to avoid technical problem in high boost ratio and low conversion rate in the step-up process. The main drawback of serially connected batteries is that each battery has different life span and if one goes bad, the whole supply will be cut off. In addition, all the batteries have to be replaced at once and the same brand has to be used so that the capacity of rechargeable batteries will match. While according to analysis of characteristic of power-voltage curve, there is no matching needed for parallel connected batteries and the number can be added or reduced at will so that it is easy to maintain and repair. Therefore, it is increasingly important to have high efficiency bi-directional converter for parallel connected batteries with low voltage.
Reference for traditional bi-directional converters are listed in [1]-[10] and their specifications, capacities, efficiencies, circuit topology and advantages/disadvantages are compared in Table 1.
TABLE 1Technical ComparisonLowHighOutputConver.CircuitRef.Vol.Vol.Capa.RatioTopologyMerits and drawbacks[1]24 V 48 V100W94%Half-bridgeMerit: High efficiency with lightload.Drawback: Can't be used forheavy load[2]12 V380 V1.6kW92%Half-bridgeMerit: Phase-shift control,soft-switchingDrawback: switching frequency20 kHz, high harmonics[3] 5 V 9 V20WStep-upCapacitorMerit: Simple, no inductor85%ChargingDrawback: low efficiencyStep-down80%[4]50 V360 V200WStep-upCapacitor + Half-Merit: Synchronous rectification91%bridgeswitchingStep-downDrawback: low power and87%efficiency[5]36 V 70 V120W91%Capacitor + BridgeMerit: Phase-shift control,soft-switchingDrawback: Complex, needs 8switches[6]48 V 72 V168W89%Capacitor + BridgeMerit: Increases efficiency for lowloadDrawback: needs 8 switches, highloss[7]24 V 24 V60WDown-StreamFlybackMerit: High efficiency with light93%Transformerload.Up-StreamDrawback: no step-up and94%step-down[8]36 V340 V800WStep-upCoupledMerit: Simple89%InductorDrawback: needs specific loadStep-down—[9]10 V288 V1.6kWStep-upCapacitor + BridgeMerit: High Step-up ratio, High94%efficiencyStep-downDrawback: Complex, needs 995%switches[10]80 V100 V200W91%Capacitor + BridgeMerit: Uses inductor as power50 VsourceDrawback: need large capacity oftransformer, and 8 switches
From Table 1, most converters utilize transformer and 4-9 power semiconductor switches. Even though a few use zero-voltage or zero-current soft-switching, switching loss and conduction loss are still increased dramatically due to the current passing through too many switches. Also, transformers are not well suited to wide range of voltage changes due to saturation of the magnetic core caused by fluctuating excited induction current and the transformer has to bear all the output power.
Reference:
    [1] D. H. Xu, C. H. Zhao, and H. F. Fan, “A PWM plus phase-shift control bidirectional DC-DC converter,” IEEE Trans. Power Electron., vol. 19, pp. 666-675, 2004.    [2] F. Z. Peng, H. Li, G. J. Su, and J. S. Lawler, “A new ZVS bidirectional DC-DC converter for fuel cell and battery application,” IEEE Trans. Power Electron., vol. 19, pp. 54-65, 2004.    [3] H. S. H. Chung, W. C. Chow, S. Y. R. Hui, and S. T. S. Lee, “Development of a switched-capacitor DC-DC converter with bidirectional power flow,” IEEE Trans. Circuits Syst., vol. 47, pp. 1383-1389, 2000.    [4] M. Jain, M. Daniele, and P. K. Jain, “A bidirectional DC-DC converter topology for low power application,” IEEE Trans. Power Electron., vol. 15, pp. 595-606, 2000.    [5] H. L. Chan, K. W. E. Cheng, and D. Sutanto, “Bidirectional phase-shifted DC-DC converter,” Electron. Letters., vol. 35, pp. 523-524, 1999.    [6] H. L. Chan, K. W. E. Cheng, and D. Sutanto, “ZCS-ZVS bi-directional phase-shifted DC-DC converter with extended load range,” IEE Proc. Electr. Power Appl., vol. 150, pp. 269-277, 2003.    [7] G. Chen, Y. S. Lee, S. Y. R. Hui, D. H. Xu, and Y. S. Wang, “Actively clamped bidirectional flyback converter,” IEEE Trans. Ind. Electron., vol. 47, pp. 770-779, 2000.    [8] C. Y. Inaba, Y. Konishi, and M. Nakaoka, “High frequency PWM controlled step-up chopper type DC-DC power converters with reduced peak switch voltage stress,” IEE Proc. Electr. Power Appl., vol. 151, pp. 47-52, 2004.    [9] K. Wang, C. Y. Lin, L. Zhu, D. Qu, F. C. Lee, and J. S. Lai, “Bi-directional DC to DC converters for fuel cell systems,” in Proc. IEEE Workshop Power Electron. Transport, 1998, pp. 47-51.    [10] Y. M. Chen, Y C. Liu, and F. Y. Wu, “Multi-input DC/DC converter based on the multiwinding transformer for renewable energy applications,” IEEE Trans. Ind. Appl., vol. 38, pp. 1096-1104, 2002.    [11] Q. Zhao and F. C. Lee, “High-efficiency, high step-up DC-DC converters,” IEEE Trans. Power Electron., vol. 18, no. 1, pp. 65-73, 2003.
The present invention uses coupled inductor bidirectional topology and only 3 switches to control bidirectional currents. Due to the characteristics of high step-up and step-down ratio, low-voltage rechargeable battery can be injected into a high-voltage dc bus to benefit down stream high-voltage load or front-end of an inverter. Since the techniques of voltage clamping, synchronous rectification, zero-voltage and zero-current techniques are used in the present convert, and the corresponding device specifications are adequately chosen, it can achieve the goal of high-efficiency bi-directional power conversion for power sources with high voltage gain.